A conventional antifuse, for example a gate oxide antifuse with a simple capacitor-like structure, can be blown, for example with an on-chip voltage generator, at a very low current. The resistance of the antifuse is typically substantially lower after the programming operation. For example, with a gate oxide antifuse, the resistance before programming is typically in the range of 109 ohms, while the resistance after the programming operation may typically be in the range of 105 ohms. For a given antifuse structure, the post-programming resistance decreases as the current during the programming operation increases.
However, when the programming current is increased beyond a certain limit, the post-programming resistance has been observed to jump back into the higher resistance range normally observed before programming. In one example of a gate oxide antifuse, the distribution of post-programming antifuse resistance becomes narrower and shifts to lower values as the programming current density increases, ultimately reaching a minimum at about 1000 A/cm2. However, as the current density increases toward 10,000 A/cm2, the distribution of the post-programming resistance becomes wider and shifts back to the higher, pre-programming values. Thus, in order to produce a post-programming resistance that can be distinguished from the pre-programming resistance, an upper limit can be set for the programming current, but such an upper limit can disadvantageously complicate the process of programming the antifuse.
It is therefore desirable to relax the aforementioned upper limit on antifuse programming current, while still producing a post-programming resistance that is distinguishable from the pre-programming resistance.
The present invention recognizes that, the increased post-programming antifuse resistance conventionally observed when a higher programming current is used, is actually a detection error that results from damage in the programming current path. The invention provides first and second independent current paths connected to the antifuse. One of the current paths can be used to program the antifuse, and the other current path can be used to detect the status of the antifuse. In this manner, the antifuse status can be correctly determined after programming, even if the programming current damages the programming current path. Thus, the aforementioned upper limit on programming current can be relaxed while still retaining acceptable antifuse status detection capability.